Method for stabilizing contact lenses

ABSTRACT

A method for stabilizing contact lenses includes providing a lens design with a nominal set of stabilization zone parameters, applying a merit function to the lens design based on balancing moments of momentum, and creating a contact lens design with improved stabilization based on the application of the merit functions to the lens design with a nominal set of stabilization zone parameters.

BACKGROUND OF THE INVENTION

Correction of certain optical defects can be accomplished by impartingnon-spherical corrective aspects to one or more surfaces of a contactlens such as cylindrical, bifocal, or multifocal characteristics. Theselenses must generally be maintained at a specific orientation while onthe eye to be effective. Maintenance of the on-eye orientation of a lenstypically is accomplished by altering the mechanical characteristics ofthe lens. Prism stabilization including decentering of the lens' frontsurface relative to the back surface, thickening of the inferior lensperiphery, forming depressions or elevations on the lens' surface, andtruncating the lens edge are examples of stabilization approaches.Additionally, dynamic stabilization has been used in which the lens isstabilized by the use of thin zones, or areas in which the thickness ofthe lens' periphery is reduced. Typically, the thin zones are located attwo regions that are symmetric about either the vertical or horizontalaxis of the lens from the vantage point of its on-eye placement.

Evaluating lens design involves making judgments concerning theperformance of the lens on-eye and then optimizing the design ifnecessary and possible. This process is typically done by clinicallyevaluating the test design in patients. However, this process is timeconsuming and expensive because it requires a significant number ofpatients to be tested since patient to patient variability must beaccounted for.

There is a continuing need for improving the stabilization of certaincontact lenses.

SUMMARY OF THE INVENTION

The invention is a contact lens designed with improved stabilizationrelative to a nominal stabilized design.

In another aspect of the invention, a method for stabilizing contactlenses incorporates a lens design with a nominal set of stabilizationzone parameters, evaluating the on-eye performance of the lens design,calculating a merit function based on this performance, and optimizingthe stabilization zone parameters by applying the merit function. Thisprocess may be performed iteratively via a virtual model (e.g., softwarebased) that simulates the effects of eye mechanics such as blinking andadjusts the stabilization scheme accordingly.

In yet another aspect of the invention, contact lenses are stabilizedaccording to a scheme in which the moment of momentum of torques actingon the lens on-eye are balanced.

In yet another aspect of the invention, contact lenses are stabilized bythe formation of one or more zones having a different thickness than theremainder of the lens and in which these zones are located on the lensessuch that the moment of momentum of torques acting on the lens when thelens is on-eye will be balanced.

In yet another aspect of the invention a contact lens has astabilization zone with the bulk of its length lying beneath thehorizontal axis of the lens.

In yet another aspect of the invention a contact lens has astabilization zone having a differing rate of change of slope (from itspeak) in one direction relative to the other.

In yet another aspect of the invention a contact lens has a different inheight profile above the horizontal axis than it does below thehorizontal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front or object view of a stabilized contact lens.

FIGS. 2A-2C are schematic representations of an eye with an insertedlens; it identifies the axis of rotation and various torques acting atthe lens.

FIG. 3 is a flowchart showing a stabilization optimization processaccording to the invention.

FIGS. 4A-4C illustrate is a front view of a stabilized lens withstabilization zones corresponding to example 1 along with correspondingcircumferential and radial thicknesses.

FIGS. 5A-5C illustrate a front view of a stabilized lens withstabilization zones corresponding to example 2 along with correspondingcircumferential and radial thicknesses.

FIGS. 6A-6C illustrate a front view of a stabilized lens withstabilization zones corresponding to example 3 along with correspondingcircumferential and radial thicknesses.

FIGS. 7A-7C illustrate a front view of a stabilized lens withstabilization zones corresponding to example 4 along with correspondingcircumferential and radial thicknesses.

FIG. 8 is a graph showing rotation speed measurements.

DETAILED DESCRIPTION

Contact lenses of this invention have designs that optimizestabilization based on balancing various forces that act on the lenses.This involves the application of a design process that balances torquesoperating on the eye, components of the eye, and ultimately thestabilized lens that is placed on the eye. Preferably, improvedstabilization is attained by starting the improvement process with anominal design that includes stabilization elements. For example, a lensdesign that has two stabilization zones that are symmetric about boththe horizontal and vertical axes running through the center is aconvenient reference from which to optimize stabilization of the lensaccording the inventive methods. By “stabilization zone” is meant anarea of the peripheral zone of the lens which has thickness valueslarger than the average thickness of the remaining areas of theperipheral zone. By “peripheral zone” is meant the area of the lenssurface circumferentially surrounding the optic zone of the lens, andextending up to but not including the edge of the lens. Anotherstabilization design that is a useful starting point is described in USPatent Publication 20050237482 which is incorporated herein by referencebut any stabilization design can be used as the nominal design that isthen optimized according to this invention. The stabilization designimprovement process can also incorporate testing the improvement withthe eye model described below, evaluating the results of the testing,and continuing iteratively with the improvement process until adesirable level of stabilization is achieved.

FIG. 1 depicts the front, or object side, surface of a stabilized lens.Lens 10 has an optical zone 11. The lens' periphery surrounds optic zone11. Two thick regions 12 are located in the periphery and arestabilization zones.

The model that is preferably used in the process to produce the newdesigns incorporates various factors and assumptions that simulate themechanical operation and their effects on lens stability. Preferably,this model is reduced to software using standard programming and codingtechniques according to well-known programming techniques. In broadoverview, the model is used in the process for designing stabilizedlenses by simulating the application of the forces described below in aprescribed number of blinks of the eye. The degree to which the lensrotates and decenters is determined accordingly. The design is thenaltered in a way that is directed to bringing rotation and/or centrationto more desirable levels. It is then subjected to the model again todetermine translation upon blinking after the pre-determined number ofblinks. The alteration of the design is accomplished by the applicationof merit functions described in more detail below.

The model assumes that the eye preferably consists of at least twospherical surface parts representing the cornea and the sclera and thatthe origin of the x-y-z coordinate axes is in the center of the sphererepresenting the cornea. More complex surfaces such as aspheric surfacesmay also be used. The base shape of the lens consists of sphericalsurface parts but the base curve radius of the lens is allowed to changefrom the center of the lens towards the edge. More than one base curvemay be used to describe the back surface. It is assumed that a lenspositioned on the eye assumes the same shape as that of the eye. Thethickness distribution of the lens need not necessarily be rotationallysymmetric and indeed is not symmetric according to some preferredembodiments of the inventive lenses. Thick zones at the edge of the lensmay be used to control the position and orientation behavior of thelens. A uniform thin film of liquid (tear film) exists between the lensand the eye, with a typical thickness of 5 μm. This tear film isreferred to as the post-lens tear film. At the lens edge the thicknessof the liquid film between the lens and eye is much smaller and isreferred to as the mucin tear film. A uniform thin film of liquid (also,tear film) with a typical thickness of 5.0 μm exists between the lensand the lower and upper eyelids and these are referred to as thepre-lens tear films. The boundaries of both the lower and the uppereyelids lie in planes having unit normal vectors in the x-y plane.Hence, the projections of these boundaries on the plane perpendicular tothe z-axis are straight lines. This assumption is also made during themotion of the eyelids. The upper eyelid exerts a uniform pressure on thecontact lens. This uniform pressure is exerted on the whole area of thecontact lens covered by the upper eyelid or on a part of this area nearthe boundary of the upper eyelid with uniform width (measured in thedirection perpendicular to the plane through the curve describing theedge of the eyelid). The lower eyelid exerts a uniform pressure on thecontact lens. This pressure is exerted on the whole area of the contactlens covered by the lower eyelid. The pressure exerted by the eyelids onthe contact lens contributes to the torque acting at the lens through anon-uniform thickness distribution (thick zone) of the contact lens,especially near the edge. The effect of this pressure on the torquesacting at the contact lens is referred to as the melon seed effect.Viscous friction exists in the post-lens tear film if the lens moveswith respect to the eye. Viscous friction also exists in the mucin tearfilm between lens edge and the eye if the lens moves with respect to theeye. Additionally, viscous friction exists in the pre-lens tear film ifthe lens moves and/or the eyelids move. Strains and stresses in the lensoccur due to the deformation of the lens. These strains and stressesresult in an elastic energy content of the lens. As the lens moves withrespect to the eye and the deformation of the lens changes, the elasticenergy content changes. The lens tends toward a position at which theelastic energy content is minimal.

The parameters describing the geometry of the eye (cornea and sclera),base shape of the lens and the motion of the eyelids are shown in FIGS.2A-2C. The motion of the lens follows from the balance of moment ofmomentum acting at the lens. Inertial effects are neglected. Then thesum of all moments acting at the lens is zero. Hence,

0={right arrow over (M)} _(l,cor) +{right arrow over (M)} _(l,muc)+{right arrow over (M)} _(l,low) +{right arrow over (M)}l,upp+{rightarrow over (M)} _(i,Ulow) +{right arrow over (M)} _(l,Upp) +{right arrowover (M)} _(l,Vupp) +{right arrow over (M)} _(ms,low) +{right arrow over(M)} _(ms,upp) +{right arrow over (M)} _(elast) +{right arrow over (M)}^(grav)

The first 4 moments are resisting torques and are linearly dependent onthe lens motion. The remaining torques are driving torques. This balanceof moment of momentum results in a non-linear first order differentialequation for the position β of the lens

${{A\left( {\overset{\_}{\beta},t} \right)}\frac{\overset{\rightarrow}{\beta}}{t}} = {{{\overset{\rightarrow}{M}}_{total}^{driving}\left( {\overset{\_}{\beta},t} \right)}.}$

This equation is solved with a fourth order Runge-Kutta integrationscheme. The positions of points on the contact lens follow from arotation around the rotation vector β(t). The rotation matrix R(t)transforming the old position of points to the current position followsfrom Rodrigues's formula

${\overset{\rightarrow}{x}}_{new} = {{\overset{\rightarrow}{x}}_{old} + {\sin \mspace{11mu} {\beta \left( {\overset{\rightarrow}{n} \times {\overset{\rightarrow}{x}}_{old}} \right)}} + {\left( {1 - {\cos \mspace{11mu} \beta}} \right)\left( {\overset{\rightarrow}{n} \times \left( {\overset{\rightarrow}{n} \times {\overset{\rightarrow}{x}}_{old}} \right)} \right)}}$${\overset{\rightarrow}{x}}_{new} = {{R(t)}{\overset{\rightarrow}{x}}_{old}}$${{where}\mspace{14mu} \overset{\rightarrow}{n}} = {{\frac{\overset{\rightarrow}{\beta}}{\overset{\rightarrow}{\beta}}\mspace{14mu} {and}\mspace{14mu} \beta} = {{\overset{\rightarrow}{\beta}}.}}$

In the numerical integration method a time-discretization is used. Thenthe motion of the lens can be seen as a number of subsequent rotations,hence at the next time step t_(n+1) the rotation matrix is

R _(n+1) =R _(Δt) R _(n)

where R_(Δt) is the rotation during the time step Δt.

The rotation matrix is decomposed into a rotation R_(α) and adecentration R_(θ) of the lens

R(t)=R _(θ)(t)R _(α)(t)

The rotation of the lens is a rotation around the centerline of thelens. The decentration is a rotation around a line in the (x, y) plane.Hence, the position of the lens is seen as a rotation {right arrow over(α)} of the lens around its centerline followed by a decentration {rightarrow over (θ)}.

In a preferred method of the invention, merit functions (MFs) based onthese relationships are fashioned to adjust and thereby improvestabilization schemes of nominal designs. These merit functions aredefined based on lens on-eye performance requirements. In a preferredembodiment the merit functions are defined but not limited to: a) lensrotation and centration performance (Equation 1), b) lens stabilityaround a resting position (Equation 2), or c) Lens rotation andcentration performance and stability around resting position (Equation3).

$\begin{matrix}{{MF}_{1} = \sqrt{{W_{R}\left( \frac{Rot}{R_{REF}} \right)}^{2} + {W_{C}\left( \frac{Cent}{C_{REF}} \right)}^{2}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

By lens rotation is meant the angular movement of the lens around itsz-axis occurring during and in between blinks. Rotation may be clockwiseor counter clockwise depending on the initial position of the lens onthe eye or the lens behavior when modeled on the eye.

By lens centration is meant the distance between the lens geometriccenter and the corneal apex. Centration is recorded in an x-y coordinatesystem in the plane of the corneal apex.

By lens stability is meant the amount of maximum lens movement in thehorizontal direction (x axis) and the vertical direction (y axis) andamount of lens rotation during the blink period. Lens stability ispreferably recorded with no lens misorientation and decentration afterthe lens reaches its final position.

Using Equation 1 as exemplary of the purpose and application of themerit function, Rot and Cent respectively describes lens performance inrotation and centration of the lens design to be optimized. R_(REF) andC_(REF) are variables describing the lens performance in rotation andcentration of the initial lens design. W_(R) and W_(C) are two weightingfactors allowing the adjustment of the contribution of one factorrelative to the other and can take values between 0 and 1. When applied,as exemplified below, these functions are best solved numerically. Theweighting factors are applied so that components of interest are givenappropriate consideration. They may be equal or one component may be ofmore interest than another. Thus, for example, if one is more concernedwith optimizing rotation than centration they would select a W_(R) thatis greater than W_(C). A stabilized design is improved when its meritfunction is decreased relative to the design that preceded it under thisconstruct. Further, it is optimized when the merit function is minimizedin such a case. Of course, one lens design may be preferable to anotherfor reasons other than stabilization so an improved stabilization maystill be undertaken according to the invention without necessarilyoptimizing the stabilization aspect of the design.

$\begin{matrix}{{MF}_{2} = \sqrt{{W_{X}\left( \frac{X_{Range}}{X_{REF}} \right)}^{2} + {W_{Y}\left( \frac{Y_{Range}}{Y_{REF}} \right)}^{2} + {W_{\theta}\left( \frac{\theta_{Range}}{\theta_{REF}} \right)}^{2}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

In Equation 2, X_(Range), Y_(Range) and θ_(Range) describe lensperformance in stability in horizontal direction, vertical direction androtation of the lens design to be optimized, X_(REF), Y_(REF) andθ_(REF) the lens performance in stability in horizontal direction,vertical direction and rotation of the initial lens design, and W_(X),W_(Y) and W_(θ) the weighting factors allowing the adjustment of thecontribution of the factors relative to each others.

$\begin{matrix}{{MF}_{3} = \sqrt{{W_{R}\left( \frac{Rot}{R_{REF}} \right)}^{2} + {W_{C}\left( \frac{Cent}{C_{REF}} \right)}^{2} + {W_{S}\left( \frac{Stab}{S_{REF}} \right)}^{2}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

In Equation 3 Rot, Cent and Stab describe lens performance in rotation,centration and stability of the lens design to be optimized, R_(REF),C_(REF) and S_(REF) the lens performance in rotation, centration andstability of the initial lens design, and R_(REF), C_(REF) and S_(REF)the weighting factors allowing the adjustment of the contribution of thefactors relative to each others.

In another embodiment merit functions include wearing comfort and canalso include stabilization zone volume, stabilization zone surface area,soft contact lens wearer awareness to stabilization zone or any otherrelevant criteria.

In further preferred embodiments the merit functions are defined fromthe following parameters in the same fashion as those set forth above:

-   -   Rotation performance:        -   Surface area below the rotation curve response        -   Time to reach resting position in rotation within +/−5.0            degrees        -   Initial rotation speed    -   Centration performance:        -   Surface area below the centration curve response        -   Time to reach resting position in centration        -   First time to reach the final resting position        -   Centration speed    -   Stability performance:        -   Magnitude of movement in horizontal direction        -   Magnitude of movement in vertical direction        -   Magnitude of rotation        -   Duration of horizontal movement        -   Duration of vertical movement        -   Duration of rotation.    -   Wearing comfort:        -   Volume of material in excess for building stabilization zone        -   Surface area covered by stabilization zone        -   Lens wearer awareness to stabilization zone

There is no limitation in the type of stabilization that can be producedby the method. Stabilization zones can be of the following type:

-   -   Symmetrical with respect to X and Y axis    -   Symmetrical with respect to X or Y-axis    -   Asymmetric with respect to both X and Y axis    -   Constant radial distance    -   Variable radial distance

Various stabilization zone parameters may be evaluated during theoptimization, including without limitation the following: zone length,peak thickness location, ramp angles on either side of the peak,circumferential tilt of the zone, and zone width. The optimizationparameters can also include lens diameter, base curve, thickness, opticzone diameter, peripheral zone width, material properties, otherparameters describing lens features.

In a preferred embodiment of the invention, two types of improvementapproaches are disclosed. In the first, a full optimization is conductedin which the model of on-eye behavior with a given iteration of thestabilization adjustment induced by the MF requires several blink cyclesuntil the lens reaches its resting position. In another embodiment, thedesign is improved during a predetermined number of blink cycles. Threeblink cycles are generally the least that will be effective in providingmeaningful stabilization improvements. In either case, the process isconducted iteratively with the application of the MF to a nominaldesign. In the case in which three blink cycles are used, an initialblink orients the lens at an angle α from the horizontal, intermediateblink where the lens is oriented at an angle β from the horizontal, andfinal blink where the lens is positioned at the resting location. In themost preferred embodiment the angle α is set to 45 degrees and the angleβ is set to 22 degrees (but both angles are not limited to thesevalues). In another embodiment the optimization process is a combinationof both approaches where a reduced number of blink cycles is preliminaryused to reach an intermediate solution then several blink cycles areused to validate that the optimization has been conducted to anacceptable degree.

FIG. 3 shows a flow chart of this improvement process. The initialstabilization zones design can be either a pre-existing design or a newdesign. Stabilization zone parameters from these designs are determined.These parameters are obtained from calculating the design performancewhen the parameters are modified around their initial values. Parametersthat give the most variations in the lens performance are preferablyselected for the optimization process. In step 1, stabilization zoneparameters are selected for consideration. These can include, forexample, magnitude of the stabilization zone (Z₀), peak location along0-180 deg. meridians (r₀), peak location angularly around the 0-180 deg.meridian (θ₀), slope above and below the peak location, the angularlength of the stabilization zone (σ₀), stabilization zones rotatedaround the peak location, and width of the stabilization zone (C_(R))among many others.

Weight W_(R) W_(C) W_(X) W_(Y) W₆ Example 1 1.00 1.00 0.50 0.50 1.70Example 2 1.00 1.00 0.50 0.50 1.70 Example 3 1.00 1.00 0.50 0.50 1 70Example 4 1.00 1.00 0.50 0.50 1.70 Performance indicator R_(REF) C_(REF)X_(range) Y_(range) θ_(range) Example1 505.110 1.100 1.03 2.65 1.88Example2 218.91 0.416 1.02 2.67 0.52 Example3 277.22 0.356 1.03 2.680.67 Example4 349.32 0.780 1.02 2.67 0.55 Merit functions % ImprovementEqu. (1) Equ. (2) Equ. (1) Equ. (2) Example1 1.414 1.643 N/A N/AExample2 0.575 1.062 59.32 35.35 Example3 0.637 1.106 54.36 32.68Example4 0.990 1.070 29.97 34.88In step 2, the lens is defined mathematically in terms of thestabilization zones parameters to arrive at an initial or nominaldesign. There is no limitation in the type of mathematic functiondescribing the stabilization zones. Stabilization zones can also bedescribed using computer generated software such as CAD applications.The mathematically described design (with defined parameters) is enteredinto the eye model in Step 3 and rotation, centration, and stabilitydata is generated as shown in Table 1). This data can then be used tomodify one or more of the stabilization parameters in optional step 4.

Table 1. Performance indicators obtained from designs of examples 1, 2,3, and 4 applied to the merit functions defined by equations (1) and(2).

The stabilization zones are modified either by reshaping, scaling,rotating, shifting, or using any other techniques to modify the currentdesign. In steps 5a-5d the modified stabilization parameters are againrun through the eye model to generate rotation, centration, andstability data for each of now modified designs. In each case ofcorresponding steps 6a-6d merit functions are created and applied toeach new design to generate new rotation, centration, and stability datain steps 7 and 8 as the lens is maneuvered (preferably throughrotation). Again, in each iteration merit functions are calculated instep 9 and checked to see if they are decreasing in step 10. A decreaseis an improvement over the previous iteration. If the merit function didnot decrease then a stabilization parameter can then be modified againin optional step 11 and the resulting modified lens design is then putback into the selection and data generation steps 7 and 8. If the meritfunction did decrease it shows an improvement in stabilization and thelens design is determined to be the final design (step 12) or otherzones are again improved in optional step 13. The invention may find itsgreatest utility in toric and multifocal lenses. Additionally, thedesigns may be useful in lenses customized to a specific individual'scorneal topography, lenses incorporating high order wave-frontaberration correction, or both. Preferably, the invention is used tostabilize toric lenses or toric multifocal lenses as, for example,disclosed in U.S. Pat. Nos. 5,652,638, 5,805,260 and 6,183,082 which areincorporated herein by reference in their entireties.

As yet another alternative, the lenses of the invention may incorporatecorrection for higher order ocular aberrations, corneal topographicdata, or both. Examples of such lenses are found in U.S. Pat. Nos.6,305,802 and 6,554,425 incorporated herein by reference in theirentireties.

The lenses of the invention may be made from any suitable lens formingmaterials for manufacturing ophthalmic lenses including, withoutlimitation, spectacle, contact, and intraocular lenses. Illustrativematerials for formation of soft contact lenses include, withoutlimitation silicone elastomers, silicone-containing macromers including,without limitation, those disclosed in U.S. Pat. Nos. 5,371,147,5,314,960, and 5,057,578 incorporated in their entireties herein byreference, hydrogels, silicone-containing hydrogels, and the like andcombinations thereof. More preferably, the surface is a siloxane, orcontains a siloxane functionality, including, without limitation,polydimethyl siloxane macromers, methacryloxypropyl polyalkyl siloxanes,and mixtures thereof, silicone hydrogel or a hydrogel, such as etafilconA.

Curing of the lens material may be carried out by any convenient method.For example, the material may be deposited within a mold and cured bythermal, irradiation, chemical, electromagnetic radiation curing and thelike and combinations thereof. Preferably, for contact lens embodiments,molding is carried out using ultraviolet light or using the fullspectrum of visible light. More specifically, the precise conditionssuitable for curing the lens material will depend on the materialselected and the lens to be formed. Suitable processes are disclosed inU.S. Pat. No. 5,540,410 incorporated herein in its entirety byreference.

The contact lenses of the invention may be produced by any convenientmethod. One such method uses an OPTOFORM™ lathe with a VARIFORM™attachment to produce mold inserts. The mold inserts in turn are used toform molds. Subsequently, a suitable liquid resin is placed between themolds followed by compression and curing of the resin to form the lensesof the invention. One ordinarily skilled in the art will recognize thatany number of known methods may be used to produce the lenses of theinvention.

The invention will now be further described with respect to thefollowing non-limiting examples.

Example 1

A contact lens having a known design for correcting the vision ofastigmatic patients is shown in FIG. 4A along with correspondingcircumferential and radial thicknesses as shown in FIGS. 4B and 4Crespectively. It was designed using conventional lens design softwarewith the following input design parameters:

-   -   Sphere power: −3.00D    -   Cylinder Power: −0.75D    -   Cylinder Axis: 180 deg    -   Lens diameter: 14.50 mm    -   Front optical zone diameter of 8.50 mm    -   Back optical zone diameter of 11.35 mm    -   Lens base curve: 8.50 mm    -   Center Thickness: 0.08 mm    -   Eye model parameters used are listed in Table 2A and 2B.

The stabilization zone is an extra thick zone added to the thicknessprofile of that lens. The initial stabilization zone is constructedusing a combination of normalized Gauss functions describing the radialand angular changes in thickness. The mathematical expression describingthe Sag of the stabilization zone in polar coordinates is:

${Z\left( {R,\theta} \right)} = {Z_{0} \cdot {{Exp}\left( {{- 0.5} \cdot \left( \frac{r - r_{0}}{\sigma_{R}} \right)^{2}} \right)} \cdot {{Exp}\left( {{- 0.5}\left( \frac{\theta - \theta_{0}}{\sigma_{0}} \right)^{2}} \right)}}$

Where Z₀ is the maximum magnitude of the stabilization zone, r₀ and θ₀the radial and angular location of the peak, and σ_(R) and σ_(θ) areparameters controlling the profile of the thickness change in the radialand angular directions.

The change in slope along the radial and angular direction is obtainedusing log-normal Gauss distributions. The equation becomes:

${Z\left( {R,\theta} \right)} = {Z_{0} \cdot {{Exp}\left( {{- 0.5} \cdot \left( \frac{{{Log}(r)} - r_{0}}{\sigma_{R}} \right)^{2}} \right)} \cdot {{Exp}\left( {{- 0.5} \cdot \left( \frac{{{Log}(\theta)} - \theta_{0}}{\sigma_{0}} \right)^{2}} \right)}}$

The design parameters controlling the stabilization zones are:

-   -   Change in magnitude of the stabilization zone (Z₀).    -   Peak location changes along 0-180 deg. meridians (r₀).    -   Peak location changes angularly around the 0-180 deg. meridian        (θ₀).    -   Slope changes above and below the peak location.    -   Change of the angular length of the stabilization zone (σ₀).    -   Stabilization zones rotated around the peak location.    -   Change in width of the stabilization zone (C_(R)) along the        0-180 deg. meridian.

The values from which the initial stabilization zone were built were:

-   -   Z₀=0.25 mm    -   r₀=5.75 mm    -   σ_(R)=0.50 mm    -   θ₀=180 degrees and 0 degrees for left and right stabilization        zones, respectively    -   σ_(θ)=25.0 degrees

The stabilization zone was then added to the original lens thicknessprofile. The final maximum lens thickness was 0.38 mm. A graphicillustration of the profile is shown in FIGS. 4B and 4C. Stabilizationzones are symmetric about both horizontal and vertical axes with slopesthat uniformly descend from peak height

TABLE 2A Initial parameters provided to the eye model. Tear film Aqueouslayer viscosity 8.30E−04 [Pa · s] Mucin layer viscosity 1.50E−03 [Pa ·s] Mucin layer thickness 3.50E−07 [m] Pre-lens tear film thickness5.00E−06 [m] Post-lens tear film thickness 5.00E−06 [m] Cornea Radius7.95E−03 [m] Eye Sclera Radius 1.15E−02 [m] geometry Visible CorneaRadius 5.82E−03 [m] File (eye geometry) [mm] Lens base curve radii8.50E−03 [m] Lens Lens transition radii 5.50E−03 [m] properties File(lens back geometry) [mm] Contact angle edge −5.00 [degree] Contact arealens edge 2.40E−05 [m2] Density of lens material 1000 [Kg/m3] Young'smodulus 280000 [N/m2] Poisson's ratio 0.48 [—] File (lens normalthickness profile) [mm] Lower eyelid lateral displacement 4.00E−03 [m]Eyelid Upper eyelid lateral displacement 3.50E−03 [m] geometry Time tocomplete downward 0.082 [s] and blink motion upper eyelid propertiesTime to complete blink 0.258 [s] Time between two blinks 3 [s] Eyelidpressure 200 [N/m2] Lower eyelid position at start of 6.35E−03 [m] blinkUpper eyelid position at start of 4.70E−03 [m] blink Width pressure bandat upper 5.00E−04 [m] eyelid edge Angle upper eyelid at start of blink−4.47 [degree] Angle lower eyelid at start of blink −2.07 [degree] Eyevelocity 2 — Gaze Eye gaze (choice in predefined 0 — direction transientgaze direction) Amplitude gaze 20 [degree] Frequency gaze 2.78 [Hz]Initial Initial rotation angle lens 0.00 [degree] position Initialdecentration in X-direction 0.00 [m] Initial decentration in Y-direction0.00 [m] Gravity Gravity 9.80 [m/s2] Simulation Number of cycles thathas to be 5 — parameters simulated Number of time steps in [0, −400 —Tdownblink] (If <0 then specified time step is used) Specified time step0.005 [second] Lens discretization in radial 20 — direction Lensdiscretization in 90 — circumferential direction

TABLE 2B Initial parameters provided to the eye model. Eye velocity 1Velocity eyelid constant 2 Velocity eyelid matching human eye velocityEye gaze (choice in predefined transient gaze direction) 0 No change ingaze direction 1 Horizontal motion 2 Vertical motion 3 Circular motion(counter clockwise) 4 Circular motion (clockwise) 5 Constant gaze inhorizontal direction 6 Constant gaze in vertical direction

The contact lens rotation and centration characteristics were determinedusing the eye model described above with initial parameters provided inTable 2. Rotation of the lens steadily decreased from about 45 deg toless than 10 degrees as the number of blinks modeled went from 0 to 20.Over the course of blinks 1-20, centration remained relatively steadyfrom about 0.06 mm to just over 0.08 mm. The resulting value of themerit function defined by equation 1 applied on the prior art lens was1.414 with W_(R)=W_(C)=1.0. This example shows the rotation, centrationand stability achieved by the lens of these parameters where maintenanceof the on-eye orientation is achieved using depressions or elevations onthe periphery of the front surface.

Example 2

A new stabilization zone was designed using the eye model andoptimization method described above and the initial design described inExample. The merit function was defined using

-   -   Surface area below the response in rotation.    -   Surface area below the response in centration.    -   Identical weight for rotation and centration, W_(R)=W_(C)=1.0.    -   The values from which the initial stabilization zone were built        were:    -   Z₀=0.25 mm    -   r₀=5.75 mm    -   σ_(R)=0.50 mm    -   θ₀=180 degrees and 0 degrees for left and right stabilization        zones, respectively    -   σ_(θ)=25.0 degrees

The stabilization zone was then added to the original lens thicknessprofile.

The stabilization zone was rotated around the peak location until thelens performance characteristics represented a significant improvementover the initial design. The rotation was obtained by applying acoordinate transformation (rotation around the peak location) on theoriginal stabilization zone coordinates:

$\left( {x,y} \right) = {\begin{bmatrix}{{Cos}(\alpha)} & {{Sin}(\alpha)} \\{{Sin}(\alpha)} & {{Cos}(\alpha)}\end{bmatrix}\left( {x_{0},y_{0}} \right)}$

Where (X₀,Y₀) were the original coordinates and (x,y) were the newcoordinates, and a the angle of rotation.

An improved stabilization design was obtained in which the finalorientation of the stabilization zone was 10.0 degrees off from thevertical with the upper portion of the stabilization oriented toward thecenter of the lens as shown in FIG. 5A along with correspondingcircumferential and radial thicknesses as shown in FIGS. 5B and 5Crespectively. Additionally, stabilization zones are not symmetric aboutthe horizontal axis. In this case, the bulk of the long dimension ofeach zone lies above the horizontal axis. The final value of the meritfunction was 0.58. Improvement on the merit function was of the order of59%. Rotation declined sharply relative to the initial stabilizationdesign. Beginning at blink 4 less than 30 degrees rotation were seenwith no rotation from blink 12 onward compared to about 40-25 degrees ofrotation seen in the initial design over the same range of blinks.Centration remained steady with less than 0.04 mm on blink 1 and lessthan 0.03 thereafter in the improved design relative to 0.06 to greaterthan 0.08 for the initial design over the same number of blink cycles.This example shows improved rotation, centration and stability ascompared with the lens of Example 1.

Example 3

A new stabilization zone was designed using the eye model andoptimization method described above and the initial design described inExample 1. The merit function was defined using

-   -   Surface area below the response in rotation.    -   Surface area below the response in centration.    -   Identical weight for rotation and centration, W_(R)=W_(C)=1.0.    -   The values from which the initial stabilization zone were built        were:    -   Z₀=0.25 mm    -   r₀=5.75 mm    -   σ_(R)=0.50 mm    -   θ₀=180 degrees and 0 degrees for left and right stabilization        zones, respectively    -   σ_(θ)=25.0 degrees

The stabilization zone was added to the original lens thickness profile.

An improved stabilization design was obtained in which the finalorientation of the stabilization zone was such that the peak location ofthe stabilization zone was changed angularly around the 0-180 deg.meridian from the geometrical center of the lens as shown in FIG. 6Aalong with corresponding circumferential and radial thicknesses as shownin FIGS. 6B and 6C respectively. Stabilization zones are no longersymmetric about the horizontal axis and the rate of change of the slopeof those zones differs in direction away from the 0-180 meridian. Thefinal value of the merit function was 0.64. Improvement on the meritfunction was of the order of 55%. Rotation declined sharply relative tothe initial stabilization design. Beginning at blink 4 less than 30degrees rotation were seen with about 10 degrees of rotation seen atblink 10 and no rotation from blink 16 onward compared to about 40-30-15degrees of rotation seen in the initial design over the same range ofblinks. Centration was less than 0.06 mm on blink 1 and less than 0.04at blink 4. Thereafter it dropped sharply, being less than 0.02 at 8blinks and zero at blink 16 relative to greater than 0.06 to greaterthan 0.07 and greater than 0.08 for the initial design over the samenumber of blink cycles. This example shows improved rotation, centrationand stability as compared with the lens of Example 1.

Example 4

A new stabilization zone was designed using the eye model andoptimization method described above and the initial design described inExample 1. The merit function was defined using

-   -   Surface area below the response in rotation.    -   Surface area below the response in centration.    -   Weight for rotation W_(R)=0.84, weight for centration        W_(θ)=1.14.    -   The values from which the initial stabilization zone were built        were:    -   Z₀=0.25 mm    -   r₀=5.75 mm    -   σ_(R)=0.50 mm    -   θ₀=1.954    -   σ_(θ)=0.14

The stabilization zone was added to the original lens thickness profile.The stabilization zone was adjusted to change the slope around the peaklocation. Peak locations remain on the 0-180 degrees meridian as shownin FIG. 7A along with corresponding circumferential and radialthicknesses as shown in FIGS. 7B and 7C respectively. Stabilizationzones are not symmetric about the horizontal axis and the rate of changeof the slope of those zones differs in direction away from peak height.This is accentuated in this case with a much more gradual decline inslope toward the bottom portion of the lens. Slope changes were obtainedusing a log-normal Gauss distribution function for describing angularlythe thickness change. The final value of the merit function was 0.86.Improvement on the merit function was of the order of 30%. Rotationdeclined moderately relative to the initial stabilization design.Beginning at blink 6 less than 30 degrees rotation were seen with about10 degrees of rotation seen at blink 12 and no rotation from blink 16onward compared to about 38-30-15 degrees of rotation seen in theinitial design over the same range of blinks. Centration was less than0.08 mm on blink 1 and less than 0.07 at blink 4. Thereafter it droppedsharply, being less than 0.05 at 8 blinks and 0.04 at blink 16 relativeto 0.06 to greater than 0.07 and 0.08 for the initial design over thesame number of blink cycles. This example shows improved rotation,centration and stability as compared with the lens of Example 1.

FIG. 8 summarizes the rotation speed versus lens orientation on eye forthe example 1, 2, 3, and 4. The initial design described in example 1has an average rotation speed of about −0.55°/sec. in the 45°-0°miss-orientation range, while designs given in examples 2, 3 and 4 havean average rotation speed above −0.70°/sec. within the samemiss-orientation range. Example 2 and 4 have higher rotation speed formiss-orientations below 15°. Both designs are more adequate for lensesrequiring single orientation on eye such as soft contact lenses designedfor high order aberration correction. These designs might requiredifferent fitting method requiring special fiducials on the frontsurface to help the patient for lens insertion. As the lens orientationon eye is unique due to the asymmetry of the stabilization and becauseof the markings on the front surface, the orientation of the lens duringinsertion should be very close to the final orientation of the lensafter it reaches its resting position. High rotation speed for smallmiss-orientation at insertion will provide faster full visioncorrection. Those designs also present better centration performanceover the design of example 3. Lens centration gets stable over a smallernumber of blinks.

1. A method for stabilizing contact lenses comprising: a) providing alens design with a nominal set of stabilization zone parameters, b)applying a merit function to the lens design based on balancing momentsof momentum, and c) creating a contact lens design with improvedstabilization based on the application of said merit functions to thelens design with a nominal set of stabilization zone parameters.
 2. Themethod of claim 1 wherein steps b and c are conducted iteratively. 3.The method of claim 2 wherein a virtual model that simulates the effectsof eye mechanics is used to validate contact lens designs.
 4. The methodof claim 3 wherein blinking is one of the eye mechanics and adjusts thestabilization scheme accordingly.
 5. A method for stabilizing contactlenses comprising a) providing a lens design with a nominal set ofstabilization zone parameters, b) evaluating the on-eye performance ofthe lens design, c) calculating a merit function based on saidperformance, and c) optimizing the stabilization zone parameters byapplying the merit function.
 6. The method of claim 5 performediteratively with a virtual model.
 7. The method of claim 6 wherein themodel simulates the effects of eye mechanics.
 8. The method of claim 7wherein said mechanics include blinking.
 9. A contact lens madeaccording the method of claim
 1. 10. A contact lens made according tothe method of claim
 2. 11. A contact lens made according the method ofclaim
 3. 12. A contact lens designed with improved stabilizationrelative to a nominal stabilized design wherein the moments of momentumare balanced.